Growth of brightest cluster galaxies via mergers since z=1

Hierarchical assembly within clusters of galaxies is tied directly to the evolution of the brightest cluster galaxies (BCGs), which dominate the stellar light in the centres of rich clusters. In this paper, we investigate the number of mergers on to BCGs in 14 X-ray-selected clusters over the redshift range 0.8 < z < 1.4 using HST imaging data. We find significant differences in the numbers of companion galaxies to BCGs between the clusters in our sample, indicating that BCGs in similar mass clusters can have very different merging histories. Within a 50 kpc radius around the BCGs we find an average of 6.45 +/- 1.15 companion galaxies with mass ratios (companion:BCG) between 1 : 1 and 1 : 20. The infalling companions show a 50/50 split between major (1 : 1-1 : 2) and minor (1 : 3-1 : 20) mergers. When compared to similar work using lower redshift clusters, these results demonstrate that both major and minor merging were more common in the past. Since the dynamical time-scales for merging on to the BCG are relatively short compared with the look-back time to z similar to 1, our results suggest that the BCG stellar mass may increase by as much as 1.8 times since z = 1. However, the growth rate of BCGs will be substantially less if stripped material from nearby companions ends up in the diffuse intracluster light.